This invention relates to a low reflectivity radome for enclosing and protecting a radar antenna, particularly the type carried by aircraft. The present invention is more particularly addressed to employing foamed plastic materials stiffened with ribs made from materials having a substantially greater dielectric constant but extending normal to the surface of such a radome in an orthogonal arrangement for very low reflectivity to minimize side-lobe clutter and side-lobe jamming.
Conventional radomes are designed on the basis of a compromise between desired electrical and structural properties. For ideal electrical performance, the beam passing to and from the radar antenna should be entirely unaffected by the radome. The only way this can be accomplished is unacceptable, namely, to eliminate the radome entirely. For airborne radar, the structural configuration of the radome is of utmost importance to not only the function of the radar itself but also the aerodynamic shape of the radome on the aircraft. Common types of radomes include conventional fiberglass A- and C-type sandwiches which are satisfactory from the weight/stiffness aspect but prohibit attainment of reflectivity lower than, for example, -25 to -30 db. Typical fiberglass sandwiches with glass cloth resin walls and honeycomb openings between them provide a nearly-ideal distribution of stiffness but unfortunately the distribution of refractivity is highly unsatisfactory. Ideally, the refractivity should be substantially at a maximum in the center of the radome to pass the microwave through the radome with a minimal reflection.
The fundamental underlying concept of radome designs for decades has been to optimize the fiberglass sandwich structure by better resins, better adhesives, and improved computer programs to predict radio-frequency performance. However, radome walls having a dielectric constant of approximately .epsilon. = 4 lying parallel to the E-field will have reflections of intolerable levels for certain electronic systems.